Plastic materials have begun being widely used as recent materials for optical lenses from the viewpoints of lightweight property, processability, stability, dyeability, high-volume production capability, reducibility in cost, etc.
Among various properties required of materials for optical lenses, it is extremely important for them to have a low specific gravity. More specifically, if an optical lens formed of a material low in specific gravity is obtained, the use of such a lens permits reduction in the weight of a lens system, which occupies an important position in optical instruments, for example, microscopes, cameras and telescopes, and spectacle lenses.
Even in plastic lens materials, therefore, there is a tendency to attempt making their specific gravity further lower so as to stress predominance over materials for inorganic glass lenses.
For example, a diethylene glycol bisallyl carbonate resin designated "CR-39" is known as a plastic material for spectacle lenses spreading widest at present. However, the specific gravity of this resin is as comparatively high as 1.31 (in terms of a value measured at 20.degree. C.; the same shall apply to the following).
Besides, plastic lens materials containing halogen atoms or sulfur atoms have recently been known as materials for high-refractive index lenses. However, such plastic lens materials also have a specific gravity as comparatively high as about 1.3 to 1.4.
Further, thermoplastic resins such as polystyrene (specific gravity: 1.02), polymethyl methacrylate (specific gravity: 1.20) and polycarbonate (specific gravity: 1.19) have been known as plastic lens materials having a comparatively low specific gravity. However, these thermoplastic resins do not have sufficient optical properties required from the viewpoint of practical use.
On the other hand, copolymers having a crosslinked structure have been introduced as plastic lens materials excellent in various performance characteristics such as heat resistance, solvent resistance and mechanical strength. For example, "NIKON LIGHT DELUXE II" and the like have been put to practical use as plastic lens materials having a triazine ring structure. The specific gravity of the plastic lens material is as comparatively low as 1.17. However, this plastic lens material cannot be yet said to be satisfactory for more reducing the weight of plastic lenses.